For carrying out photochemical reactions to produce hydrogen and oxygen from water with metallic catalysts, several processes have been disclosed. These are lanthanide-like photcatalysts such as, for example, NaTaO3:La, catalysts from rare earth metals, such as R2Ti2O7 (R=Y, rare earth metal), or TiO2-derived semiconductor systems, so-called tandem cells, wherein up to now no use of silicides has been disclosed for the application according to the title.
The processes for production of hydrogen and oxygen from water are based on reduction and/or oxidation of water with semiconductor materials and light. Such processes are also named water splitting processes. The hitherto disclosed processes use UV light. Although in some cases an appreciable development of hydrogen and oxygen has been found, the required illumination conditions are not suitable for solar-based application of the process. In addition, the production of the catalysts is laborious and requires the use of uneconomically high temperatures, starting from expensive basic materials of extremely high purity. Furthermore, for performing the aforementioned processes the use of very pure (triple distilled) water is required. In most cases there is no mention with respect to long-term applicability and the related stability of the catalysts.
The only promising approach so far employs silicide powder wherein the semiconductor itself splits the water into hydrogen and oxygen gas in presence of light. Oxygen has to be separately removed from the system. All these systems can be used only to split water and not for simultaneously or separately occurring production of electric power (photovoltaic principle).
Silicides have so far not been used for the configuration of photovoltaic systems. Only in individual cases (i.e., IrSi2 and beta-FeSi2) electrical and optical properties of films were measured.